GB2270415A - Anisotropically loaded helix assembly for a travelling-wave tube - Google Patents

Abstract

An anisotropically loaded helix assembly for use in a travelling-wave tube includes a helix circuit 12 concentrically supported within a conductive cylindrical housing 10 by a plurality of dielectric support members 16, preferably composed of aluminium nitride, and on which conductive material 18 is deposited directly, as by silk screening, on to the dielectric support members in order to provide an anisotropic load. As shown in Fig. 2, the conductive coating 18, eg formed of copper or gold, is U-shaped and formed on the three sides of the support member 18 not contacting the helix. Alternatively, Fig. 5, the support members 36 may be T-shaped with the conductive coating 38 formed in separate parts. In other arrangements, eg. Fig. 7 (not shown), the conductive coating may be shaped, eg tapered, to provide a variable impedance. A sub-assembly of a helix attached to three dielectric support rods having the conductive coating may be assembled in a cylindrical housing by elastically deforming the housing in three regions to permit unhindered insertion of the sub-assembly therein, and upon removal of the deforming forces the cylindrical housing returns to its nominal shape thus compressing, and securing, the sub-assembly therein. <IMAGE>

Description

to 1 2.270415 ANISOTROPICALLY LOADED HELIX ASSEMBLY FOR A TRAVELLING-WAVE

TUBE AND METHOD OF MAKING SAME The present invention relates to an anisotropically 5 loaded helix or equivalent assembly for use in a travellingwave tube and to a method of manufacturing such assembly.

As will be appreciated by a person skilled in the art, the phase velocity within a travelling-wave tube (TWT) varies with frequency. As the frequency is reduced, the number of helix turns per wavelength increases. As a result, the coupling of electric and magnetic fields between the turns of the helix changes, and there is a cancellation of the magnetic flux between the adjacent turns. Consequently, the inductance of the TWT helix decreases, allowing the wave velocity to increase. Additionally, as the frequency is reduced and the number of helix turns per wavelength increases, the electric field created by the helix extends further from the helix.

In order to slow down the wave velocity, and co'nsequently the phase velocity, within a TWT, a metal housing is concentrically placed around the helix. Since low frequency signals create electric fields that extend further from the TWT helix, than do high frequency signals, a metal housing can be used to decrease wave velocity at low frequencies while having very little effect on high frequency operation. The effectiveness of the metal housing on slowing down wave velocity at low frequencies is proportional to the distance of the metal housing from the 2 TWT helix. Unfortunately, to bring the metal housing closer to the TWT helix also has the accompanying disadvantage of decreasing circuit interaction impedance, which decreases the gain and efficiency of the TWT. Ideally, if a metal housing could be created that conducted only in its axial direction, the disadvantageous effect of the metal housing on circuit impedance could be avoided because no circumferential currents from the TWT helix would flow into the surrounding metal shell housing.

In practice, an axially conducting shell housing is approximated by a technique commonly referred to as anisotropic loading. In anisotropic loading, a metal shell housing is concentrically supported around the TWT helix by a plurality of dielectric supports. Shaped conductive members are then attached to the inside diameter of the surrounding metal housing and are extended down toward the TWT helix, in between the dielectric supports. The conductive members are commonly called loading vanes and the dispersion of the TWT helix is controlled by shape and position of the loading vanes relative to both the central helix and the surrounding metal housing. In addition to the presence of the conductive loading vanes, wave velocity within the TWT helix is also affected by the presence of the dielectric supports that separate the TWT helix from the surrounding metal housing. Wave velocity is inversely proportional to the square root of the dielectric constant of the material from which the supports are produced. Consequently, when dielectric material is added in the region surrounding the TWT helix, the wave velocity within the TWT helix decreases.

Figures la, lb and lc of the accompanying drawings show cross-sectional views of three typical prior art anisotropic loading configurations for a TWT. Referring to Figure la. a metal housing 10 is concentrically supported around a TWT helix 12 by a plurality of symmetrically disposed dielectric supports 14. on the inner wall of the metal housing, are supported a plurality of conductive 3 loading vanes 16. The dielectric supports 14 and the loading vanes 16 are held in place by being brazed, adhesively bonded or mechanically fastened to the inner diameter wall of the metal housing. Similarly, the dielectric supports 14 are brazed, adhesively bonded or mechanically fastened to the TWT helix.

Referring to Figure lb, the TWT shown has a plurality of shaped metal clips 18 used as the loading vanes. The clips 18 also act to hold the dielectric members 14 symmetrically in place around the TWT helix 12. In Figure lc a TWT is shown having large solid loading vanes 20. As with the embodiment of Figure la, the solid loading vanes must be brazed, mechanically fastened or adhesively attached to the metal shell 10.

is The embodiments of the prior art do have some substantial disadvantages. The manufacture of an anisotropic loaded TWT assembly is usually a labour intensive and expensive process and in prior art assembly methods, the insertion of the dielectric supports, and the

TWT helix, into the metal shell may be accomplished by heating the metal shell and TWT helix in a vacuum oven.

Furthermore, the mass of the loading vanes usually produce an excessive overall shell loading which cause a reduced interaction impedance.

It will be recognised by a person skilled in the art, that such hot insertion manufacturing techniques requires the TWT assembly to cool for hours before it can be tested. Additionally, prior art configurations use large conductive elements to form the loading vanes. These conductive elements, either in the form of metal clips or vanes, are expensive to manufacture and require very exacting manufacturing techniques to assemble the TWT helix, metal housing and loading vanes in a concentric orientation. Additionally, the use of separate loading vanes and dielectric supports have made prior art TWT's susceptible to sudden changes in acceleration and other shocks which may dislodge a loading vane element or alter the TWT9s

4 concentric construction.

Another disadvantage of many prior art TWT's is the material used to construct the dielectric supports that separates the TWT helix from the metal shell. In prior art

TWT's, the dielectric supports are often constructed of beryllia or boron nitride. Both beryllia and boron nitride are expensive materials. Furthermore, the thermal conductivity of both beryllia and boron nitride is limited, creating theoretical limitations on the performance characteristics of a TWT.

In view of the above stated problems in the prior art, it is therefor a primary object of the present invention to provide an anisotropically loaded helix assembly for use in a TWT,, and a method for making such an assembly that produces a TWT that is lower in cost, easier to manufacture, more resistive to shock, has a higher temperature range and operates more efficiently than other common anisotropically loaded TWT1s.

According to the present invention there is provided an anisotropically loaded assembly for use in a travellingwave tube including a helix or any alternative slow wave structure circuit, concentrically supported within a conductive cylindrical housing by a plurality of dielectric support members. The anisotropic load is provided in the form of conductive material deposited directly on to the dielectric support members and may comprise a strip formed by using a known technique such as silk screening, photolithographic controlled vapour deposition or the like. By depositing a strip of conductive material directly onto the dielectric support members, the labour and cost of manufacturing and assembling separated anisotropic loading vanes is removed, thereby allowing the assembly to be more easily and less expensively manufactured. Furthermore, part tolerances can be more readily controlled, thereby reducing variations from one device to another. By reducing variation between manufactured components, the yield of the TWT can be increased. Additionally, by utilizing conductive material selectively deposited directly onto the dielectric support members, the performance of a TWT can be improved. In an anisotropically loaded TWT there exist regions where there is little or no inductance interaction in between the helix circuit and the surrounding anisotropic load. Such positions occur at the beginning and end of the helix circuit as well as in the centre of loss patterns. In the present invention, the strip of conductive material, deposited onto each dielectric support member, is only electrically coupled to the surrounding cylindrical housing in areas where there is little or no impendence interaction. By constructing the anisotropic loads in such a manner, the exchange of circumferential currents from the helix circuit to the cylindrical housing is reduced. thereby improving the overall axial vane loading, bandwidth range and efficiency of the TWT.

The use of anisotropic loads directly deposited onto the dielectric support members has the added advantage of allowing the impendence of the anisotropic load to be readily varied, per unit length, so as to provide impendence matching to the helix circuit. The impedance of the anisotropic load may be effected by selectively shaping the conductive strips so as to follow the needed impedance value profile.

The present invention can effect further advantages over the prior art through the material and shape into which the dielectric support members are fabricated. In the present invention, the dielectric support members are preferably formed from aluminum nitride. The use of aluminum nitride provides an advantage over prior art boron nitride and beryllium (Beo) dielectrics, in that aluminum, nitride is less expensive, has a higher thermal conductivity and effects a higher gain per inch within the TWT.

It is well known in the art that wave velocity within an unloaded TWT helix circuit increases as the frequency of the input signal decreases. The variations in wave velocity causes phase variations which reduce the 6 overall gain of the TWT. The use of dielectrics adjacent to the helix circuit decreases the wave velocity within the TWT helix circuit. Consequently, the use of a specifically shaped dielectric support member can compensate for the 5 increase in wave velocity caused by a low frequency signal. Low frequency signals produce loose electromagnetic field lines, thereby producing a limited flux within the helix circuit and allowing the wave velocity to increase. As the signal frequency increases, electromagnetic field lines become more concentrated near the helix circuit and the wave velocity slows down. By creating a dielectric support member that is wide near the cylindrical housing and narrow near the helix circuit, a device is formed that affects the electromagnetic field of low frequency signals disproportionately to high frequency signals. Since the dielectric material affects electromagnetic fields so as to slow wave velocity, the dielectric support member can be appropriately shaped to counteract the increase in wave velocity produced by a low frequency signal.

The present invention also provides a method for assembling an anisotropically loaded helix assembly. The method preferably includes elastically deforming the cylindrical housing so that the helix circuit and dielectric support members can be placed therein. Once positioned within the cylindrical housing, the cylindrical housing is returned to its nominal shape, completing the assembly in a time and cost efficient manner. The metallized dielectric support members can be used in any other variety of TWT assembly methods. Regardless of the method of assembly, assembly is simplified by the user of less component parts.

For a better understanding of the present invention exemplary embodiments thereof will now be described in conjunction with Figures 2 to 8 of the accompanying drawings, in which:

Figure 2 is a perspective exploded view of one preferred embodiment of an anisotropically loaded travelling-wave tube helix and shell assembly according to 7 the invention; Figure 3 is a cross-sectional view of the travelling-wave tube helix and shell assembly as depicted in Figure 2, viewed along section lines 33; 5 Figure 4 is a perspective exploded view of a second preferred embodiment of an anisotropically loaded travelling-wave tube helix and shell assembly according to the invention; Figure 5 is a cross- sectional view of the travelling-wave tube helix and shell assembly as depicted in Figure 4, viewed along section line 4-4; Figure 6 is a perspective view of an alternative construction of dielectric support member for an assembly in accordance with the present invention; is Figure 7 is a perspective view of a second alternative construction of dielectric support member for an assembly in accordance with the present invention; Figure 8a is an end view of a preferred form of travelling-wave tube shell elastically deformed from circular by three symmetrically disposed forces; Figure 8b shows a travelling-wave tube helix assembly positioned within the travelling-wave tube shell, shown in Figure 8a; and, Figure 8c shows the final position of the travelling-wave tube helix of Figure 8b within the shell once the deforming forces acting on the shell have been removed.

Referring to Figure 2 and Figure 3 of the drawings, there is shown a travelling-wave tube helix and shell assembly 11, used in the construction of an anisotropically loaded travelling-wave tube (TWT). The helix and shell assembly 11 comprises a conductive helix 12 concentrically positioned within a metal cylinder 10. The helix 12 is supported within the cylinder 10 by a plurality of dielectric support members 16 formed from aluminum nitride (AIN) or any other dielectric material capable of being metallised. The AIN support members 16 are symmetrically 8 disposed around the helix 12, thereby being one hundred and twenty degrees apart for the three AIN support members 16 in the shown embodiment. Each of the AIN support members 16 is subjected to a metallising procedure wherein a selectively shaped layer of conductive material 18 is deposited onto the AIN support members 16. The layer of conductive material 18 can be deposited onto the AIN support members 16 using known metallisation techniques, but is preferably applied to the AIN support members 16 using known silk screening application procedures. In the shown embodiment the conductive material 18 is deposited along three adjacent sides of the AIN support member 16 so as to form a substantially U-shaped anisotropic load across the length of each AIN support member 16. By depositing the conductive material 18 directly onto the AIN support members 16, anisotropic loads are created that- do not need to be assembled as separate members, thereby reducing the time and labour required during the assembly procedure.

Additionally, by depositing the anisotropic loads directly onto the AIN support members 16,, the overall assembly becomes more resistive to physical shock, which would dislodge separately formed loading vanes as used in prior art assemblies. To assemble the TWT helix and shell assembly 11, the AIN support members 16 with the helix 12 must be positioned within the metal cylinder 10. The AIN support members 16, with their integrally formed conductive loading vanes are af f ixed to the helix 12 using a non conductive, high temperature adhesive 19. such as EASTMAN cement type 4655. The attachment of the AIN support members 16 to the metal cylinder 10 is effected without the use of adhesives or mechanical fasteners, as will later be described when detailing the preferred manufacturing method.

Typically, prior art anisotropically loaded travelling-wave tubes utilise boron nitride (BN) or beryllia material in f orming the dielectric support members that separate the helix 12 from the metal cylinder 10. However, the use of AIN support members 16 in place of BN or beryllia 9 support members has advantages that improve the state of the art for TWTs. The presence of dielectric support members, within an anisotropically loaded TWT, cause the wave velocity within the helix 12 to decrease. Wave velocity is inversely proportional to the square root of the dielectric constant of the support members. The use of AIN support members 16 in place of BN or beryllia support members creates an increase in efficiency in the performance of the TWT in the form of a higher gains per inch due to a lower loss tangent. Additionally, AIN support members 16 have a higher thermal conductivity than do similarly formed BN support members, and AIN support members 16 have a thermal conductivity that is higher than beryllia at temperatures greater than 1000 C. Consequently, AIN aupport members 16 dissipate heat more rapidly than BN and beryllia support members and thus provide a thermal advantage in the operating temperature range. Furthermore, the cost of AIN support members 16 is far less than the cost of BN or beryllia support members, thereby providing a cost advantage.

- Referring to Figure 4 and Figure 5, an alternative embodiment of TWT helix and shell assembly 30 is shown. The shown embodiment of the TWT helix and shell assembly 30 includes a conductive helix assembly 32 concentrically positioned within a metal cylinder 10. Supporting the helix assembly 32 within the metal cylinder 10 are a plurality of support member assemblies 34. The support member assemblies 34 are symmetrically disposed around the helix assembly 32, being spaced apart by one hundred and twenty degrees in the case of the illustrated three support member assembly embodiments.

The helix assembly 32 comprises two helices which are oppositely wound and connected in the centre of a loss pattern, or a singular split helix. Such two-piece helical windings are known and widely used in the art. Each of the support member assemblies 34 comprises a shaped dielectric base 36 on which is formed a strip of conductive material 38. The shaped dielectric base 36 is preferably made of aluminum nitride for the reason stated in the description of Figures 2 and 3, however, it should be understood that the dielectric base 36 can be formed of any dielectric material including boron nitride or beryllia. In the present invention separate conductive loading vanes of prior art configurations are replaced by strips of conductive material herein called loading strips 38 that can be deposited on either side of the dielectric base 36. The loading strips

38 may be formed from copper, gold or any other highly conductive material. The loading strips 38 are deposited along the length of the dielectric base 36 so that the main body of each loading strip 38 is supported between the helix assembly 32 and the metal cylinder 10. The loading strips 38 may be deposited on the dielectric base 36 utilising any known method such as sputter deposition or the like, but the loading strips 38 are preferably applied using known silk screening techniques.

The various loading strips 38 can be formed to mimic 20 prior art loading vanes by forming the loading strips 38 so that they define a substantially Ushaped layer and contact the metal cylinder 10 along the entire length of the dielectric base. However, as can be seen from Figures 4 and 5, the assembly is preferably constructed so that the loading strips 38 only contact the surrounding metal cylinder at three discrete contact points 42, 44, 46. The first and last contact points 42, 46 correspond in position to the forward and distant ends of the helix assembly 32. At-these points the interaction between the loading strips 38 and the electromagnetic field of the helix assembly 32 is at a minimum. The centre contact point 44 corresponds in position to the split between the two helices, which also corresponds to the centre point within a loss pattern. In a centre of a loss pattern there is also a reduction in interaction between the anisotropic load and the electromagnetic field produced byu the helix assembly 32. By only coupling the loading strips 38 to the metal cylinder

11 at points of low electromagnetic interaction, the corresponding circumferential currents, created by the coupling, also occur at points of low electromagnetic interaction. Consequently, circumferential currents are reduced and the anisotropic loading ef f ects of the TWT helix and shell assembly 30 are improved, resulting in a TWT with an improved operational bandwidth.

The coupling of the loading strips 38 to the metal cylinder can be accomplished by increasing the width of the loading strips 38 so that the deposited material covers the section of the dielectric base 36 that contacts the metal cylinder 10. The overlap of the deposited material on the surface of the dielectric base 36 becomes the discrete contact points 42, 44, 46. Alternatively, a loss conductive carbon pattern can be deposited onto the dielectric base 36 at a point corresponding to the discrete contact point 44 for a single loss pattern TWT. The conductive carbon deposit then acts to couple the loading strips 38 to the metal cylinder. The low resistance connection created by the carbon deposit only allows azimuthal shell currents to be generated at the loss sections, wherein the negative effects of the shell currents are minimal.

By using loading strips 38, deposited onto the dielectric base 36, instead of individually formed discrete conductive loading vanes, the overall outer shell loading is reduced. Consequently, the reduced interaction impedance created by the metal cylinder 10 is reduced and the TWT can operate more efficiently. Furthermore, the use of loading strips 38, deposited onto the dielectric base 36, represents a reduction in parts, labour and expense as compared to prior art assemblies utilising individually formed loading vanes.

In addition to the improved efficiency obtained by selectively coupling the loading strips 38 to the metal cylinder 14, the operational efficiency of the TWT can be further improved by selectively shaping both the dielectric base 36 and the loading strips 38 deposited onto the 12 dielectric base 36. Still referring to Figures 4 and 5, it can be seen that the dielectric base 36 has a substantially T-shaped cross section. As was previously explained, as the signal frequency within the helix assembly 32 decreases, the electromagnetic field lines produced by the helix assembly propagate away from the windings of the helix so that there is a reduction in flux, and the wave velocity of the signal increases. Adversely, the presence of dielectric material within the electromagnetic field of the helix assembly 32 causes the wave velocity of a signal to decrease. Consequently a low frequency signal and the presence of a dielectric material cause opposite effects on the wave velocity with the helix assembly 32. Consequently, the shape of the the dielectric base 36 can be specifically formed so as to compensate for the decrease in wave velocity caused by a low frequency signal, thereby increasing the operating parameters in which the TWT can efficiently operate. As a signal frequency decreases, the electromagnetic field lines created by the helix assembly 32 move further away from the helix assembly 32. In order to affect the wave velocity caused by a low frequency signal, without substantially affecting the wave velocity of a high frequency signal, the presence of the dielectric material is maximised in areas that are primarily affected by the electromagnetic field of a low frequency signal. Similarly, the presence of the dielectric material is minimised in areas primarily affected by the electromagnetic field of a high frequency signal. it is for this reason that the shown dielectric base 36 has a thin stem region 40 that contacts the helix assembly 32.

The length of the stem region 40 corresponds to the mean range of the electromagnetic field created by a high frequency signal. The head region 51 of the dielectric base 36 is much wider than the stem region 40 and corresponds to the mean range of the electromagnetic field created by a low frequency signal. In this way, the dielectric base 36 can compensate for some of the phase velocity variations caused by low frequency signal. In view of the above disclosure,

13 it should be apparent to a person skilled in the art that the shape of the dielectric base 36 is governed by the particular wave velocity variations for a given TWT application. For example, if for a given TWT the position of the electromagnetic field lines were in a linear proportional relationship to the signal frequency, the dielectric bases 36 may be formed with linear sloped walls so as to compensate for the effects of the low frequency signal on wave velocity as shown in Figure 7 described below. Similarly, if the position of the electromagnetic field lines were in an exponential relationship to the signal frequency, the dielectric bases 36 may also include an exponential change in thickness so as to compensate for the changes in wave velocity. Such an exponential change in thickness is shown in Figure 5 as contour 50 connecting the head region 51 of the dielectric base 36 to the stem region 40.

Referring now to Figure 6, a simple form of a support member assembly 90 is shown. In this construction of support member assembly, loading strips 92 are formed on either side of a dielectric base 94. The loading strips 92 are then connected at a single point by a conductive carbon pattern 96. By connecting the loading strips 96 at a single point, a single loss pattern can be created for a TWT.

Referring to Figure 7, yet another alternative form of support member assembly 52 is shown. The support member 52 includes a dielectric base 54 having a substantially v shaped cross-section. Formed on the side of the dielectric base 54 is a conductive loading strip 56, deposited in the manner previously described. However, in the construction of Figure 7, the loading strip 56 is not uniformly formed. The loading strip 56 is formed to vary in impedance, per unit length, as it progresses along the length of the dielectric base 54. The variations of impedance as a function of length can be preformed in many known manners and may include a taper within the loading strip 56 as shown in section 58 or a formed shape created as part of the 14 loading strip 56 as shown in section 60. The impedance of the loading strip 56 is formed to improve impedance match between the helix assembly and the loading strips 56 as well as to improve the interaction at the band edges of the helix 5 circuit, to increase bandwidth or band centres.

Regardless of the particular construction of support member assembly utilised, the helix assembly is supported by support member assemblies within a metal cylinder 10. In the past, the assembly of the helix assembly and support member assemblies within the metal cylinder 10 was a time consuming and labour intensive operation. Referring finally to Figures 8a through 8c, a new method of assembling is shown that reduces the amount of labour, time and expense involved within the assembly procedure and produces a more reliable TWT helix and shell assembly. In Figure 8a there is shown a metal cylinder 10, being elastically deformed by the influence of three symmetrically disposed forces (shown by arrows 70,72,74). When at rest the metal cylinder 10 would have a nominal radius Rl and would have a circular cross-section shown by broken lines 68. The forces applied to the meal cylinder 10 elastically deform it from its nominal shape. Since there are three forces 70, 72, 74 acting on the metal cylinder 10 at symmetrically opposed positions, the metal cylinder 10 is deformed into a generally triangular shape having three round apexes 76, 78, 80 occurring between the various applied forces. Each apex 76, 78, 80 is now a distance D1 from the centre of the metal cylinder, where D1 is larger than radius Rl.

In Figure 8b there is shown a loaded helix assembly 82 positioned within the elastically deformed metal cylinder 10. The helix assembly 82 includes the central helix 84 around which are symmetrically positioned three support member assemblies 86. The support member assemblies 86 are attached to the central helix 84 with adhesive 88. The surface 90 of each support member assembly 82, opposite the helix 84, is curved to match the inner diameter of the metal cylinder 10 in its nominal shape. The distance of each surface 90 is at the radius Rl from the centre of the helix, which is slightly larger than the nominal radius of the metal cylinder 10. Since the inner diameter of the metal cylinder 10 and the loaded helix assembly 82 are close in size, an interference fit occurs when the loaded helix assembly 82 is advanced into the metal cylinder 10. The interference fitis removed by the deformation of the metal cylinder 10. When the metal cylinder 10 is deformed, three apexes 76, 78, 80 occur in its configuration. The three support member assemblies 86 are aligned with the three apexes 76, 78, 80 and the loaded helix assembly 82 is inserted into the metal cylinder 10. Once the loaded helix assembly 82 is loaded into the metal cylinder 10 the deforming force is removed and the metal cylinder 10 returns to its nominal shape. By returning to its nominal shape, the metal cylinder 10 traps the loaded helix assembly 82 into place (see Figure 8c). The loading helix assembly 82 is prevented from being removed from the metal cylinder 10 by the interference fit between the metal cylinder 10 and the compression of the metal cylinder 10 around the loaded helix assembly 82 caused by the interference fit.

By elastically deforming the metal cylinder 10, inserting the loaded helix assembly 82 into the metal cylinder and letting the metal cylinder 10 return to its nominal shape, a multitude of advantages are had over prior art assembly methods. The described deformation method of construction automatically concentrically aligns the loaded helix assembly 82 with the metal cylinder, thereby eliminating labour, time and costs. The described deformation method of construction also allows the completed helix and shell assembly to be immediately advanced to the next level of assembly without waiting for the components to cool or have the loaded helix assembly 82 otherwise cure to the metal cylinder 10. Furthermore, since the assembly of the loaded helix assembly 82 to the metal cylinder 10 is accomplished without adhesives or mechanical fasteners, an improved resistance to mechanical shock is realised over the 16 existing prior art.

17

Claims (22)

1. An anisotropically loaded assembly for use within a travelling-wave tube, comprising a helix-circuit, or other slow wave structure supported within a conductive housing by a plurality of dielectric support members, wherein conductive material is directly deposited on to the dielectric support members creating an anisotropic load.

2. An assembly according to claim 1, wherein the conductive material is selectively coupled to the conductive housing at at least one discrete point thereby reducing the flow of an induced circumferential current from the helix circuit into the conductive housing.

3. An assembly according to Claim 2, including at least one strong interaction region wherein current is induced in the conductive material by the helix circuit and at least one weak interaction region wherein there is a substantial reduction in the induction interaction between the helix circuit and the conductive material, with the discrete point being positioned within the weak interaction region.

4. An assembly according to any preceding claim, wherein the conductive material is deposited on each dielectric support member with a varied impendance, corresponding to the changes of impedance of the helix circuit above which the conductive material is positioned so as to promote impedance matching between the conductive material and the helix circuit.

5. An assembly according to any preceding claim, wherein the dielectric support members are shaped so as to interact with electromagnetic field lines produced by a low frequency signal passing through the helix circuit in such a manner as to compensate for an increase in wave velocity within the helix circuit caused by the low frequency signal.

6. An assembly according to any preceding claim, wherein the dielectric support members include a narrow region, proximate the helix circuit, and a wide region, proximate the conductive housing, whereby the shape of the 18 dielectric support members, between the narrow region and said wide region corresponds to the change in electromagnetic field lines produced by the helix circuit within a given range of frequencies in such a manner as to promote a constant wave velocity within the helix circuit across the range of frequencies.

7. An assembly according to claim 6, wherein the dielectric support members have a substantially T-shaped profile having a stem section of a substantially constant width extending from the helix circuit with the stem section abruptly expanding to a wider head section proximate the conductive housing.

8. An assembly according to claim 4, or claim 5,6 or 7 dependent on claim 4, wherein the conductive material is deposited on each of the dielectric support members in a tapered pattern so as to produce the varied impedance.

9. An assembly according to any preceding claim, wherein the conductive material is deposited on the dielectric support members by a silk screen process. 20

10. An assembly according to any preceding claim, wherein the dielectric support members are formed of aluminum nitride.

11. An assembly according to claim 3, wherein the helix circuit includes a loss pattern wherein the direction of a winding forming the helix circuit is reversed or severed, the conductive material being coupled to the conductive housing at a position corresponding to the centre of the loss pattern.

12. An assembly according to any preceding claim, wherein the conductive material is coupled to the conductive housing at positions corresponding to the beginning and end of the helix circuit.

13. An assembly according to claim 3, wherein a carbon pattern is deposited on the dielectric support members at the at least one discrete point, the carbon pattern coupling the dielectric support members to the conductive housing.

19

14. An anisotropically loaded helix assembly for use in a travelling-wave tube substantially as hereinbefore described with reference to Figures 2 to 8 of the accompanying drawings.

15. A method of manufacturing an anisotropically loaded assembly for use within a travelling-wave tube, comprising the steps of depositing conductive material on to a plurality of dielectric support members creating a desired anisotropic load, attaching the dielectric support members to a helix circuit or other slow wave structure forming a sub-assembly of a predetermined configuration and inserting the sub-assembly into a conductive cylindrical housing such that the conductive material is selectively coupled to the cylindrical housing.

is

16. A method according to claim 15, wherein the step of depositing conductive material includes metallising the dielectric support members by applying the conductive material to the dielectric support members with a silk screen process, whereby the conductive material is selectively deposited on the dielectric support members in predetermined configurations.

17. A method according to claim 15 or claim 16, wherein the predetermined configuration of the sub-assembly has a dimensional interference with the cylindrical housing, preventing the passage of the sub-assembly into the housing and wherein the step of inserting the sub-assembly includes elastically deforming the cylindrical housing by selectively applying a deforming force to the cylindrical housing, whereby the cylindrical housing expands to remove the interference, placing the sub-assembly within the elastically deformed cylindrical housing and removing the deforming force such that the cylindrical housing returns to a nominal position, compressing the sub- assembly therein.

18. A method according to claim 15, 16, or 17, wherein the step of attaching the dielectric support members to the helix circuit includes adhesively attaching three dielectric support members to the helix circuit in a symmetrically disposed manner to form the sub-assembly with a substantially triangular profile, wherein each dielectric support comprises an apex within the triangular profile.

19. A method according to claims 17 and 18, wherein the step of elastically deforming the cylindrical housing includes applying the deforming force to three positions around the cylindrical housing, such that the cylindrical housing expands in between the three positions. allowing for the passage of the sub-assembly therein.

20. A method according to any of claims 15 to 19, further including the step of forming the dielectric support members into shapes such that each dielectric support member interacts with electromagnetic field lines, produced by the helix circuit in such a manner as to promote a substantially constant wave velocity within the helix circuit across a given range of frequencies.

21. A method according to any of claims 15 to 20, wherein the plurality of dielectric support members are formed from aluminum. nitride. 20

22. A method of manufacturing an anisotropically loaded helix assembly for use in a travelling-wave tube substantially as hereinbefore described with reference to Figures 2 to 8 of the accompanying drawings. 25